An alkali polymerization process of .omega.-lactam in which the .omega.-lactam is polymerized by heating in the presence of a strong alkali catalyst and a polymerization accelerator is known. Polyamide produced by the alkali polymerization process has advantages over those polyamides produced by polymerizing .omega.-lactam in the presence of water in that the tensile strength and flexural strength are great, the water absorption is low, and the heat distortion temperature is high. However, it is not desirable because its elongation is small and impact strength low. Thus, several attempts have been made to improve the disadvantages of low impact strength and small elongation while making the best use of the advantages of the alkali polymerization process.
For example, a method of copolymerizing .epsilon.-caprolactam with .omega.-laurinlactam (see Japanese Pat. Publication No. 13754/68), a method of copolymerizing .epsilon.-caprolactam with .omega.-laurinlactam or .omega.-caprinlactam in the presence of N-alkylpyrrolidone (see Japanese Pat. Publication No. 22388/72), a method of polymerizing .omega.-lactam in the presence of high molecular weight polyalkylene glycol (see Japanese Patent Application (OPI) No. 72394/74 (corresponding to German Patent Application (OLS) No. 2,248,664) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), a method of polymerizing .omega.-lactam in the presence of polybutadiene having hydroxyl groups at the main chain teminals (see Japanese Patent Publication No. 15319/77), and a method of polymerizing .omega.-lactam in the presence of N-alkylpyrrolidone and at least one compound of liquid polyolefin, polypropylene glycol, and silicone oil which are compatible with the N-alkylpyrrolidone (see Japanese Patent Publication No. 39244/80) have been proposed.
However, these methods except the one disclosed in Japanese Patent Publication No. 15319/77 in which .omega.-lactam is polymerized in the presence of polybutadiene containing hydroxyl groups at the terminals of the main chain thereof can produce only those polyamides having an impact strength (Izod impact strength) of at most about 10 kg.multidot.cm/cm. Thus, it cannot be said that the methods have succeeded in improving the impact strength.
On the other hand, the method disclosed in Japanese Patent Publication No. 15319/77 has received attention as a method of greatly improving Izod impact strength. In accordance with this method, however, a sufficient amount of polybutadiene to improve impact strength is added by the one shot process and, therefore, unreacted polybutadiene remains, causing various problems; e.g., the unreacted polybutadiene comes out onto the surface of the polyamide (bleeding), and the surface of the polyamide is very tacky and has a bad appearance. In some cases, the bleeding occurs during the polymerization. Thus, it cammot be said that the method disclosed in Japanese Patent Publication No. 15319/77 is sufficiently satisfactory for practical use.